Speculative, I realize, but this smells a little bit like they are bracing us for more classic stuff from other campaign settings to be shoehorned into the Forgotten Realms. I really, really, don't want to see Mordenkainen hanging out with Elminster in Shadowdale hiring a party of adventurers to go on a quest to defeat the Dragon Kings from Dark Sun because they tried and failed to conquer the world in Athas so now they're trying it on Toril. ...yawn...
Oh, and Keraptis, having failed to steal the magic weapons Wave, Whelm, and Blackrazor on Oerth has now moved to Toril and is building a new underground volcanic dungeon in Mt. Hotanow in the Sword Coast. I can see it now... The White Plumes of Mt. Hotanow.
If the plan is to appeal to nostalgia, the only people who are going to care about the nostalgic elements are also the same people that will hate that they're ripping the elements from their original habitat.
Lets not get hung up on the exact dictionary meaning of a single word again, please, folks. There's too much actual interesting stuff to discuss here. First person to post a dictionary definition of the word loses the internet.
Takhisis/Tiamat and Bahamut/Paladine is an old Planescape era piece of lore. Same god worshiped differently on different worlds. Never outright STATED that was true, but it was heavily implied. (Same was true for Tharizdun/Elder Elemental Eye).
Of course, Dragonlance took 30 some turns to make it untrue, but its not something he pulled out of his kiester.
I like the idea of a shared universe and used to love spelljammering around avoiding the flying pig cavalry too. I just worry that in doing so each setting loses a little bit of it's flavour. Dark Sun especially but others too.They go to all the trouble of creating a way in the DMG of saying different worlds feel different, I hope they remember it when they're making multiversal adventures. It also feels a little like they feel the realms never had any good bad guys so they're bringing others over.
I'm not gonna go out and say they're going to ruin it though. I'll wait and see. They could blow me away. They've done it with multiversal stuff before (Vecna Lives!).